But it isn't enough for the world's greatest power - greatness measured in every traditional dimension of economic, political, and military power - to limit its strategic focus to crushing only one method (terrorism) employed by one radical fundamentalist network (Al Qaeda). Shouldn't we have a greater and nobler purpose in the 21st-century world?

Our new century is dynamic in four revolutionary ways: globalization, information, sovereignty, and conflict. Globalization and information technology are revolutionizing international markets and finance, and transforming whole economies and societies. They in turn are eroding traditional nation-state sovereignty and compromising the state's ability to provide economic and physical security. Weakening of the state's monopoly on violence has led to the transformation of war and fundamental alterations in the nature of conflict.

Neither an ad hoc approach - "We'll deal with these issues as they arise" - nor the Bush administration's "war on terrorism" is an adequate framework for defining the role of the US in the years ahead. The European nation-state is giving way to the United Europe, thus making restoration of the Atlantic Alliance problematic.

The Chinese and Indian economic explosions, national redefinition in Japan, and a nuclear North Korea all require fresh US policies on Asia. Elsewhere, failed states, proliferation of weapons of mass destruction, epidemics, mass migrations, and global warming - the new agenda of the century - all require closer international collaboration and probably new international institutions.

A grand strategy is simply the application of a nation's powers to the achievement of larger purposes. I would argue we have three such purposes: to ensure security (for ourselves and, where possible, for others), to expand opportunity, and to promote liberal democracy around the world. And to achieve them, we can harness three powers - economic, political, and military - far superior to anyone else's. Our economy is larger than the next four or five largest national economies combined. We have an unrivaled diplomatic and political network. And soon we will spend more on our military than the rest of the world combined.

But we also have a fourth power, shared by few if any other great nations in history. That power is contained in our founding principles, the constitutional statement of who we are, what we believe, and how we have chosen to govern ourselves. The idea that government exists to protect, not oppress, the individual has an enormous power not fully understood by most Americans, who take this principle for granted from birth. Far more nations will follow us because of the power of this ideal than the might of all our weapons.

But the power of principle is complex: It is our greatest strategic asset and our greatest national constraint. Our principles are the predominant reason we are admired in the world, but on those occasions when we fail to live up to these principles, we are weakened accordingly.

In a more perfect world, the 2004 presidential campaign would be an ideal occasion for the candidates and their parties to present to the American people a new grand strategy for this new century. If such a strategy is not put forward, the current vacuum of purpose and belief will only continue to grow, inviting further destructive political polarization and incivility.

Ominously, this vacuum has already inspired proponents of America-as-empire to emerge, almost unchallenged. Some have argued that we are, in fact, already either a benign empire or a stealthy empire that must conceal its methods and purposes. Some so-called neoconservatives in the Bush administration have evoked Woodrow Wilson for the purpose of making the US the missionary of democracy, neglecting the important truth that Wilson's methods were internationalist and peaceful, not unilateralist and militaristic.

Throughout history, empires have been characterized by domination, political control, centralized administration, and rule by force. If the US is to become an empire, a warning must be heard: Becoming an empire, we will no longer be a republic. Throughout history, and today, the two are incompatible. Not by accident has the rise of the theory of empire coincided with the temptation to set aside our principles.

To preserve our republic, we must advance a new, alternative grand strategy that addresses this revolutionary century, one compatible with our powers and with the great principles upon which our nation was founded.